High bypass turbofan gas turbine engines for powering an aircraft in flight include a fan disposed within a nacelle powered by a core engine for generating thrust to power the aircraft. The fan includes a plurality of circumferentially spaced apart fan blades having tips disposed closely adjacent to an annular shroud supported at the inner circumference of the fan nacelle. The shroud is typically formed of conventional abradable material so that occasional rubbing of the fan blade tips thereagainst does not damage the fan blades. However, such tip rubs increase the clearance between the blade tips and the remaining fan shroud which decreases fan efficiency and resulting performance of the engine.
Accordingly, the fan shroud must be repaired or replaced in order to return the fan and engine to optimum efficiency. In one conventional design, the abradable material of the fan shroud is permanently bonded to the inside surface of a fan casing, and, therefore, the refurbishment thereof requires removal of the old material, by machining for example, which is time consuming and costly. And, the engine must be typically removed from the aircraft wing, and the fan module disassembled therefrom in order to allow access to the fan shroud for its removal, which further complicates the refurbishment process.
In another conventional design, the abradable fan shroud is mechanically joined to the fan casing using nuts and bolts so that it is more readily removable for servicing. However, the use of fasteners such as nuts and bolts as compared to adhesives for joining the fan shroud to the fan casing introduces additional parts, the liberation thereof which must be prevented for preventing foreign object damage to the fan and engine.
More specifically, since a considerable number of mating nuts and bolts must be used to rigidly secure the fan shroud to the fan casing to prevent undesirable vibratory response thereof, for example, even the loss of one such nut or bolt during assembly or operation of the engine could lead to significant damage of the engine by its ingestion therein. A nut or bolt which is dropped and lost in the engine during assembly may find its way into the rotating components thereof and cause damage. Or, if a nut or bolt loosens during operation of the engine and is liberated into the engine it too may also cause damage to the engine.
Accordingly, nuts are typically captured by a nut plate fixedly joined in the fan casing which prevents their liberation when the mating bolt is not joined thereto. And, the bolt itself upon being joined to a respective nut may be locked thereto by conventional means including suitable adhesives, locking nuts, or locking wires. Such conventional locking arrangements, however, do not prevent the loss of a bolt, for example by being dropped by maintenance personnel during the assembly process.